Hydrophobic coatings are becoming increasingly popular in numerous applications, such as windows, TV screens, DVD disks, cooking utensils, clothing, medical instruments etc because they are easy to clean and have low adhesive properties. Generally, a hydrophobic material or coating is characterised by a static contact angle of water (θ) of 90° or above. Hydrophobic polymeric materials such as poly(tetrafluorethene) (PTFE) or polypropylene (PP) have been available for decades. These materials suffer from a limited hydrophobicity, as well as inferior mechanical properties as compared to engineering materials or highly crosslinked coatings. For instance, PP has a static contact angle of water of roughly 100° whereas PTFE, which is amongst the most hydrophobic polymeric material known, has a static contact angle of water of roughly 112°.
Some hydrophobic coatings are being referred to in the art as super-hydrophobic coatings. Super-hydrophobic coatings are generally defined by a static water contact angle above 140°.
Surfaces with super-hydrophobic properties are found in nature, for example the lotus leaf or cabbage leaf. The waxes secreted onto the leaf's rough surface reduce the adhesion of water and contaminating particles to the leaf. Water droplets deposited on the leaf simply roll off, gathering dirt particles and cleaning the leaf in the process.
An enhanced hydrophobicity of coatings has been obtained via inclusion of micron-sized spherical particles in a silicone-based paint or polyolefin-based spray (BASF Press release Oct. 28, 2002, P345e, Dr Karin Elbl-Weiser, Lotusan, Nature news service/Macmillan Magazines Ltd 2002). These suspensions are applied as paint or from a spray, yet suffer from a lack in mechanical robustness. The abrasion resistance of such coatings is low and thus the coatings need to be reapplied after a short period of time to maintain the hydrophobic functionality of the surface. Additionally, the coating scatters light in the visible range, this effectively results in an opaque and optically non-transparent coating.
In U.S. Pat. No. 6,068,911, Hitachi described super-hydrophobic coatings based also on the principle of surface roughness prepared via UV curing of resins containing non-reactive nano-particles and fluoropolymers. Their coating formulation consists of at least two solvents, evaporation of the most volatile solvent drives the fluoropolymer to the surface, making it hydrophobic. The presence of the inert non-reactive nano-particles results in surface roughness and the overall coating exhibits superhydrophobicity. As this technology is based on the evaporation of an organic solvent to create surface roughness during processing, kinetics will play a role in this process. Also, the hardness, durability and abrasion resistance of the coating, leaves better performance to be desired.
Another approach is to use a non-abrasion-resistant layer that is continuously replenished from a reservoir of mobile fluor-containing agents in an immobile matrix layer with on lop a vapour-deposited top layer of inorganic material which has a large degree of roughness and cracks (WO 01/92179). The concept is that the fluoropolymers diffuse through the inorganic layer and cover the surface, thus forming a regenerative surface layer. This results in hard, optically clear surfaces with a high water contact angle and very low roll-off angle. However, the production of such complex structures via vapour deposition is very time-consuming and laborious, and the area size that can be coated is limited. Also, the release and washing away of the mobile fluoropolymers is environmentally not desirable.
One object of the invention is to provide a superhydrophobic coating having a static contact angle greater than 140° that is easy to produce, has reproducible quality, and which has very good mechanical properties.
Surprisingly this object is achieved by a superhydrophobic film or coating, comprising,                a) primary particles,        b) secondary particles adhering to the surface of the primary particles and having an average diameter that is smaller than the average diameter of the primary particles,        c) a hydrophobic upper surface layer covering at least partly the surface of the secondary particles and adhered to that surface,wherein the secondary particles are adhered to the surface of the primary particles by covalent chemical bonds.An advantage of the coating according to the invention is that it is possible to produce the coating according to the invention with a well-defined and constant quality.        
A further advantage of the coating according to the invention is that the coating is highly wear resistant and/or scratch resistant.
Yet a further advantage of the coating according to the invention is that the coating according to the invention is easy to produce.
Yet a further advantage of the coating according to the invention is that it is possible that the coating does not comprise any fluoro atoms.
Instead of using a hydrophobic upper surface layer on top of the layer comprising the raspberry particles, it is also possible to use different kind of upper surface layers, for example a hydrophilic layer, a layer having absorbing properties, for example for absorbing a smell or spreading a fragrance, having a catalytic activity, for example for oxidatively eliminating air pollutants, etc. It is even possible that the layer adhering to the surface of the secondary particles is not present at all.